DEsign pressure rating (DP) for Simonton 5500, BFRich etc.

I have been researching on the web and the discussion boards various windows and the costs involved. I checked out Simonton 5500 yesterday at a store in Central Jersey and I liked them. The other windows I liked are BFRich (local installer recommendation) and Schuco (very expensive). Here is a question. I noticed that the DP rating of the Simonton 5500 is only DP-25 while the DP rating of the BFRich DP-50. The U-factor, SHGC and other factors are comparable. Did anyone else look at the DP factor? WHy is there is a difference like this?
Any help will be greatly appreciated.

You can up a Simonton double-hung to DP-55 or higher by adding the metal reinforcement option. Also, the DP rating differs based on the dimensions of the window, so be sure and compare DP ratings between windows of the same size and with similar glass packages.